Seth Henry Neddermeyer (September 16, 1907 – January 29, 1988) was an American physicist who co-discovered themuon, and later championed theimplosion-type nuclear weapon while working on theManhattan Project at theLos Alamos Laboratory duringWorld War II.

Seth Neddermeyer
Neddermeyer's ID badge photo fromLos Alamos
Born	Seth Henry Neddermeyer (1907-09-16)September 16, 1907 Richmond, Michigan, United States
Died	January 29, 1988(1988-01-29) (aged 80) Seattle, Washington, United States
Education	University of Olivet Stanford University (AB) California Institute of Technology (MS,PhD)
Known for	Co-discoverer of themuon Implosion-typeatomic bomb
Awards	Enrico Fermi award (1982)
Scientific career
Fields	Physics
Institutions	National Bureau of Standards Los Alamos Laboratory University of Washington
Thesis	The absorption of high energy electrons (1935)
Doctoral advisor	Carl D. Anderson

Seth Henry Neddermeyer was born inRichmond, Michigan, on September 16, 1907.^[1] He attendedOlivet College, a small college that his mother, older sister, and uncle had also attended,^[2] for two years before his family moved to California. He transferred toStanford University, from which received hisBachelor of Arts (AB) degree in 1929.^[1] His interest in physics was inspired by the work ofRobert A. Millikan, and he enrolled in graduate school atCalifornia Institute of Technology (Caltech),^[2] where he wrote his 1935PhD thesis on "The absorption of high energy electrons",^[3][4] under the supervision ofCarl D. Anderson. He confirmed the theory espoused byNiels Bohr for this process. He also noted large radiative energy losses of electrons inlead, in agreement with the theory propounded byHans Bethe andWalter Heitler.^[1]

Neddermeyer contributed to the research which led to the 1932 discovery of thepositron,^[1] for which Anderson was awarded theNobel Prize in Physics in 1936.^[5] That year, Neddermeyer and Anderson discovered themuon, usingcloud chamber measurements ofcosmic rays. Their discovery predatedHideki Yukawa's 1935 theory ofmesons that postulated the particle as mediating the nuclear force. Anderson and Neddermeyer collaborated with Millikan in high altitude studies of cosmic rays, which confirmedRobert Oppenheimer's theory that theair showers produced in the atmosphere by cosmic rays contained electrons.^[1] They also obtained the first evidence thatgamma rays can generate positrons.^[3]

In early 1941, withWorld War II raging in Europe but the United States not yet a belligerent, Neddermeyer joined a team led byCharles C. Lauritsen andWilliam A. Fowler at the Department of Terrestrial Magnetism at theCarnegie Institution of Washington, and then at theNational Bureau of Standards in Washington, D.C., that worked on the photoelectricproximity fuze.^[1][2] After this work was completed, Neddermeyer was recruited by Oppenheimer to work at theManhattan Project'sLos Alamos Laboratory.^[1] Neddermeyer was an early advocate for the development of animplosion technique for assembling acritical mass in an atomic bomb.^[6] Although implosion was suggested byRichard Tolman as early as 1942 and discussed in the introductory lectures given to Los Alamos scientists byRobert Serber, Neddermeyer was one of the first to urge its full development.^[6][7] Unable to find much initial enthusiasm for the concept among his fellow Los Alamos scientists, Neddermeyer presented the first substantial technical analysis of implosion in late April 1943. Oppenheimer considered this to be the beginning of implosion research at Los Alamos.^[8]

Though many remained unimpressed, Oppenheimer appointed Neddermeyer the head of a new group to test implosion.^[9] His group became the E-5 (Implosion) Group, which was part ofCaptainWilliam S. Parsons' E Division.^[10] Agun-type nuclear weapon was the preferred method, but implosion research constituted a backup.^[11] Neddermeyer embarked on an intensive series of experiments testing cylindrical implosions. The result was a series of distorted shapes.^[10] Progress was made; Neddermeyer and a member of his team,Hugh Bradner, along withJames L. Tuck from theBritish Mission, conceived the idea ofexplosive lenses, in which shaped charges are used to focus the force of an explosion.^[6] Nevertheless, seemingly unsolvable problems withshock wave uniformity brought progress on implosion to a crawl.^[10]

By September 1943, Neddermeyer's team had grown from five people to fifty. That month,John von Neumann came to Los Alamos at Oppenheimer's request. Von Neumann was impressed by the implosion concept and, working withEdward Teller, an old friend, made a series of suggestions. Von Neumann was able to create a sound mathematical model of implosion, enabling Neddermeyer to present a proposal for a greatly expanded research program.Edwin McMillan andIsidor Isaac Rabi recommended thatGeorge Kistiakowsky, who had a specialized knowledge in the precision use of explosives, be brought in to help the program.^[12][13] In February 1944, Kistiakowsky became Parsons' deputy for implosion.^[14]

In April 1944, tests on the first sample ofplutonium that had been produced with neutrons in a nuclear reactor revealed that reactor-bred plutonium contained five times moreplutonium-240 than that hitherto produced incyclotrons. This unwanted isotope that spontaneously decayed and produced neutrons promised to cause apredetonation without sufficiently quick critical mass assembly. It now became apparent that only implosion would work for practical plutonium bombs; a powerful enough gun could not be constructed small enough to be carried in an aircraft, and plutonium-240 was even more difficult to separate from plutonium-239 than the isotopes of uranium that were giving the rest of the Manhattan Project such difficulties. Plutonium was unusable unless implosion worked, but only plutonium could be produced in quantities that would allow regular production of atomic bombs. Thus, the implosion technique now suddenly stood as the key to production of nuclear weapons.^[15]

In mid-June 1944, a report from Kistiakowsky to Oppenheimer detailing dysfunctionality within the implosion team led to the ousting of Neddermeyer.^[16] He was replaced as the head of the E-5 Group by Kistiakowsky on June 15, 1944, but remained a technical adviser to the implosion program, with group leader status.^[17] Neddermeyer was said to have been much embittered by this episode.^[16] In Oppenheimer's August 1944 reorganization of the Los Alamos Laboratory, Neddermeyer's group was renamed X-1, withNorris Bradbury as group leader.^[15] The implosion method championed by Neddermeyer was used in the first atom bomb exploded (in theTrinity test), theFat Man bomb dropped onNagasaki, and almost all modern nuclear weapons.^[18] Kistiakowsky later insisted that "the real invention should be given full credit to [Seth] Neddermeyer" (sic).^[19]

In 1946, after World War II ended, Neddermeyer left Los Alamos to become anassociate professor at theUniversity of Washington, where he would spend the rest of his career. In due course he became a full professor.^[20] He resumed his studies of cosmic rays using a cloud chamber and a new device that he invented to measure the speed of charged particles known as a "chronotron". He was particularly interested in the properties of the muon, and conducted experiments with muons atSLAC. He participated in theDUMAND Project, for which he helped design large-scale underwaterneutrino detectors.^[1] Neddermeyer became interested inparapsychology, insisting, in spite of the skepticism of many colleagues, that it warranted proper scientific investigation.^[1][2] He retired in 1973, becoming aprofessor emeritus,^[20] but he continued his research activities for as long as his health permitted. He was afflicted withParkinson's disease.^[21]

In 1982, he was presented with theDepartment of Energy'sEnrico Fermi award. His citation read:

For participating in the discovery of the positron, for his share in the discovery of the muon, the first of the subatomic particles; for his invention of the implosion technique for assembling nuclear explosives; and for his ingenuity, foresight, and perseverance in finding solutions for what at first seemed to be unsolvable engineering difficulties.^[22]

In later life, Neddermeyer was sometimes troubled by the nuclear weapons he had helped to invent.^[1] He told an interviewer in 1983:

I get so overwhelmed by a feeling of terrible guilt when I think about the history of the bomb. I'm terribly worried now about the current world situation. What the hell can we do about it?^[21]

Neddermeyer died inSeattle on January 29, 1988,^[1] from complications of Parkinson's disease.^[23]

Neddermeyer is portrayed byColin Bennett in the 1980 BBC seriesOppenheimer, by Joe D'Angerio inFat Man and Little Boy, and byDevon Bostick in theChristopher Nolan-directed filmOppenheimer (2023).^[24]

• Hoddeson, Lillian; Henriksen, Paul W.; Meade, Roger A.;Westfall, Catherine L. (1993).Critical Assembly: A Technical History of Los Alamos During the Oppenheimer Years, 1943–1945. New York: Cambridge University Press.ISBN 0-521-44132-3.OCLC 26764320.
• Rhodes, Richard (1986).The Making of the Atomic Bomb. London: Simon & Schuster.ISBN 0-671-44133-7.
• Serber, Robert (1942).The Los Alamos Primer: The First Lectures on How to Build an Atomic Bomb. University of California Press.ISBN 0-520-07576-5.{{cite book}}:ISBN / Date incompatibility (help)